Neutrinos are perhaps the most elusive and mysterious of known
fundamental particles. They steal energy from nuclear beta decay, from the
sun, from nuclear reactors, from distant supernovas. An average neutrino from
a beta decay could travel through solid lead a light-year thick without
interacting. Neutrinos are prime suspects for "dark matter", the mysterious
substance that is responsible for most of the mass in our universe. And, as
previously reported in this column [Analog - September-92], there is a
growing body of evidence that electron neutrinos may be "tachyons", particles
that always travel faster than the velocity of light and that slow down
when given more kinetic energy. But that is another story.

In this AV column we will have a look at the DUMAND project, a new $10 million
detector funded by the US Department of Energy for the detection of ultra-high
energy neutrinos. DUMAND stands for Deep Underwater Muon
And Neutrino Detector. It is now under construction in
Hawaii and will come into operation in 1993-94. It is to be placed almost 3
miles deep on a level stretch of Pacific Ocean bottom about 18 miles west of
Keahole Point on the Island of Hawaii. Floats anchored to the ocean bottom
about 40 meters apart and arranged in an octagon around a central junction
point will support nine long vertical strings of sensitive light detectors.
DUMAND will be connected to its land-based laboratory by bundles of fiber
optics cables. AT&T will lay the cables to shore, and the US Navy's manned
deep submersible DSV Sea Cliff will be used to connect, service, and repair the
parts of the detector.

But before getting into the details of construction, let's focus on the primary
question: "What is DUMAND for?" Briefly, it's for the creation of the new
science of high-energy neutrino astronomy, for looking where no one has looked
before, for finding the spots in the sky where the most energetic processes in
the universe are taking place.

Our universe contain a small number of very special "objects" that deliver to
our upper atmosphere a "rain" of very high energy particles: protons, heavier
nuclei, electrons, photons, and neutrinos. These high energy particles from
space are called cosmic rays. The details of the processes that produce cosmic
rays are not well understood, but we are coming to realize that these very high
energies are ultimately the result of the direct conversion of mass to energy
near the event horizons of black holes.

We would like to map the sky for the sources of cosmic rays, to learn where
they and what they are. Of the cosmic ray particles listed above, only the
electrically neutral photons and neutrinos are useful for "back-tracking" to
their sources. The other particles, protons, nuclei, and electrons, have
electrical charges that cause them to be deflected in random ways as they pass
through the magnetic fields of the galaxy, the solar system and the earth.
This scrambles their incoming direction so that it cannot be related to the
direction of the source. To locate the cosmic hot spots, therefore, photons or
neutrinos must be used.

Photons, of course, are the mainstay of conventional astronomy. Photons in the
microwave, infrared, visible, ultraviolet, and x-ray regions of the
electromagnetic spectrum have been used by astronomers to map the universe.
There have also been studies of the sky with gamma rays, photons with energies
of about 0.5 MeV or higher. But as the photon energy rises, detection becomes
more difficult and direction information more elusive. Gamma rays interact too
much with Earth's atmosphere, so all studies of cosmic gamma rays must be done
in space or using high altitude balloons. The sizes for gamma rays detectors
also depend on the gamma ray energy, with very high energy gamma rays requiring
extremely large detectors. These constraints have, up to now, severely
limited gamma ray astronomy.

But if detecting gamma rays from space is difficult, detecting neutrinos is
even more difficult because the interaction with matter of neutrinos is about
10-7 times weaker than the interactions of gamma rays. If gamma
rays interact too much with the atmosphere, neutrinos interact far too little.
Typically neutrino detectors must be placed deep underground, where neutrinos
can easily reach the detector but other particles are blocked by the shielding
of the earth itself. Neutrinos have only been directly detected in a few
experiments, all of which have required large quantities of matter and
elaborate detection schemes. For example the underground solar neutrino
detector located deep underground in the Homestake gold mine, where 15-18 MeV
neutrinos from the sun were first detected, used about 380 tons of
per-chloro-ethylene cleaning fluid as its detection medium. [See my columns
about neutrino detection in the 05/86 and 09/92 issues of Analog].

The DUMAND detector is designed to detect cosmic ray mu-neutrinos with energies
in excess of 1 TeV (1012 electron-volts of energy) as they pass
through sea water. Although a neutrino has zero rest mass (or perhaps nearly
zero), a 1 TeV neutrino has a mass due to its energy that is greater than the
mass of 1000 hydrogen atoms. When such a mu-neutrino (electric charge=0) has a
hard collision with a down quark (charge=-1/3) in a nucleus there is some
probability that the neutrino and quark will exchange a W boson, with the
result that the electric charge of the neutrino drops by one unit while the
charge of the quark increases by one unit. The mu-neutrino thus becomes a muon
(a mu lepton with electric charge = -1) and the down quark becomes an up quark
(charge=+2/3). The newly created muon keeps essentially all of the energy of
its parent neutrino, but it is now electrically charged. It will have a gamma
factor (or mass-increase factor) of about 10,000 and a velocity only 6 parts in
109 less than the velocity of light in vacuum. But in sea water
visible light travels only about 3/4 of its speed in vacuum. Therefore, the 1
TeV muon, newly made from the cosmic ray neutrino, will be traveling about 33%
faster than the speed of visible light in water.

When an airplane exceeds the speed of sound in air (breaks the sound barrier),
it makes a shock wave that is popularly known as a "sonic boom". Similarly,
when an electrically charged particle exceeds the speed of light in a
transparent medium like water, it makes an electromagnetic shock wave. This
shock wave is called Cêrenkov radiation, a wave front of blue light that
spreads out in a cone from the track of the superluminal charged particle. The
cone of Cêrenkov light has a characteristic direction that can be
analyzed to determine the direction of the incoming muon (and hence the
incoming mu-neutrino) to a directional accuracy of about 1o.

In a conventional high energy physics experiment, a Cêrenkov detector
might be made from a slab of transparent plastic optically coupled to a
photomultiplier tube. In DUMAND the plastic slab is replaced by 2,000,000
tons of sea water optically coupled to 216 hemispherical 15" diameter
photomultiplier tubes, each housed in a 16" spherical pressure vessel that can
sustain the water pressure of 100 atmospheres present in the ocean depths where
the detector is located.

About half of DUMAND's funding, about $4.8 million, comes from the US
Department of Energy. The other half, in the form of the photomultiplier tubes
(PMTs) and fast electronics, key components of the detector, will be the
contributions of Japanese and European collaborators. About half of the PMTs
will be made by the Hamamatsu Corporation of Japan and will be conventional
"venetian blind" photomultipliers custom made for DUMAND to achieve the
required specifications of sensitivity and timing. The European half of the
PMTs will be made by the Phillips Corporation. The Phillips PMT uses an
innovative 2 component design, a large image intensifier coupled to a small 2"
photomultiplier. Each of the PMT types has some advantages, and a mix of the
two types in the detector brings additional benefits.

A light sensitive detector like DUMAND must be placed in a dark environment,
because ambient light is a source of background. Fortunately, essentially no
daylight can penetrate the ocean to a depth of 3 miles, where the DUMAND array
is located, and the principal light sources will be from bioluminescence and
from Cêrenkov light from 40K radioactive decays in the sea
water. Neither of these is a problem, as demonstrated in November, 1987, when
a small DUMAND prototype string of PMTs and associated hardware was tested in
the ocean near Hawaii at depths down to 4 km.

One significant problem of DUMAND is the position calibration of the detector
strings, which hang above the ocean bottom on float-supported cables. Currents
in the ocean depths can cause significant movement of the cables that could, if
not taken into account, lead to significant errors in interpreting the
Cêrenkov light from energetic muons. The DUMAND experimenters solve this
problem by surrounding the detector with sonar broadcasters producing chirping
pulses of sound that are picked up by microphones placed along the detector
strings. This locates each microphone on each string to an accuracy of a few
centimeters and eliminates potential errors from the motion of the strings.

Another source of background in DUMAND comes from muons produced in the upper
atmosphere by cosmic ray protons, nuclei, electrons, and gamma rays. Some of
these muons have enormous energies and can, with some probability, penetrate
the ocean even to a depth of 3 miles. These high energy particles may be
interesting in their own right, but they are not predominately produced by
neutrinos, the particles of primary interest. Fortunately, there is an
excellent way of distinguishing neutrino-generated muons from other cosmic-ray
generated muons. If the muons are observed to pass upwards or sideways through
the detector, they can only come from neutrinos that have passed through the
bulk of the earth as neutral particles before being converted to a muon in a
collision with a quark. A muon traveling on the same path would have been
absorbed by the earth. The muons from the upper atmosphere, on the other hand,
must travel downward through the detector. All upward going muons must be the
product of neutrino events.

What cosmic cataclysms can produce neutrinos with such enormous energies? This
is a subject of some speculation in the astrophysics community. Recent
calculations have predicted that active galactic nuclei, the power source for
quasars and other high energy phenomena, can produce enough primary neutrinos
to make thousands of ultra-high energy neutrino events per year in the DUMAND
detector. Even using more conservative estimates of the cosmic neutrino flux,
the DUMAND collaboration expects about 80 neutrino events per year at energies
above 10 TeV and 300 events per year at energies above 100 GeV, which is about
the threshold of sensitivity for the detector.

DUMAND is a delightful scientific adventure, an initiative that will show us a
new aspect of nature, a technological foray that combines the forefront
techniques of electro- optics, microelectronics, communications, high energy
physics, and oceanography. And, from the point of view of the experimenters,
the shores of Hawaii will be a wonderful spot from which to explore the
mysteries of the universe.